As long as your image is not thumbnail size, we can usually make it work. If your image was taken from a cell phone or consumer digital camera, it will work great. In technical terms, any image that is 72 pixels per inch or greater, and has the height and width both greater than 1000 pixels, will work almost all the time.
Popular online services like Facebook, Instagram, Google Plus, and others will often reduce the file size of images uploaded there. It is recommended to find the original image instead of using an image that has been uploaded to social media. However, in many cases we can still make it work.
Here is our definitive article on image size when making a poster